The Nkd EF-hand domain modulates divergent wnt signaling outputs in zebrafish

Nkd1 functions downstream of Axin2 to attenuate Wnt signaling

Wnt signaling is a crucial developmental pathway involved in early development as well as stem-cell maintenance in adults and its misregulation leads to numerous diseases. Thus, understanding the regulation of this pathway becomes vitally important. Axin2 and Nkd1 are widely utilized negative feedback regulators in Wnt signaling where Axin2 functions to destabilize cytoplasmic β-catenin, and Nkd1 functions to inhibit the nuclear localization of β-catenin. Here, we set out to further understand how Axin2 and Nkd1 regulate Wnt signaling by creating axin2gh1/gh1, nkd1gh2/gh2 single mutants and axin2gh1/gh1;nkd1gh2/gh2 double mutant zebrafish using sgRNA/Cas9. All three Wnt regulator mutants were viable and had impaired heart looping, neuromast migration defects, and behavior abnormalities in common, but there were no signs of synergy in the axin2gh1/gh1;nkd1gh2/gh2 double mutants. Further, Wnt target gene expression by qRT-PCR and RNA-seq, and protein expression by mass spectrometry demonstrated that the double axin2gh1/gh1;nkd1gh2/gh2 mutant resembled the nkd1gh2/gh2 phenotype demonstrating that Nkd1 functions downstream of Axin2. In support of this, the data further demonstrates that Axin2 uniquely alters the properties of β-catenin-dependent transcription having novel readouts of Wnt activity compared with nkd1gh2/gh2 or the axin2gh1/gh1;nkd1gh2/gh2 double mutant. We also investigated the sensitivity of the Wnt regulator mutants to exacerbated Wnt signaling, where the single mutants displayed characteristic heightened Wnt sensitivity, resulting in an eyeless phenotype. Surprisingly, this phenotype was rescued in the double mutant, where we speculate that cross-talk between Wnt/β-catenin and Wnt/Planar Cell Polarity pathways could lead to altered Wnt signaling in some scenarios. Collectively, the data emphasizes both the commonality and the complexity in the feedback regulation of Wnt signaling.

Feedback control of Wnt signaling based on ultrastable histidine cluster co-aggregation between Naked/NKD and Axin

Feedback control is a universal feature of cell signaling pathways. Naked/NKD is a widely conserved feedback regulator of Wnt signaling which controls animal development and tissue homeostasis. Naked/NKD destabilizes Dishevelled, which assembles Wnt signalosomes to inhibit the β-catenin destruction complex via recruitment of Axin. Here, we discover that the molecular mechanism underlying Naked/NKD function relies on its assembly into ultra-stable decameric core aggregates via its conserved C-terminal histidine cluster (HisC). HisC aggregation is facilitated by Dishevelled and depends on accumulation of Naked/NKD during prolonged Wnt stimulation. Naked/NKD HisC cores co-aggregate with a conserved histidine cluster within Axin, to destabilize it along with Dishevelled, possibly via the autophagy receptor p62, which binds to HisC aggregates. Consistent with this, attenuated Wnt responses are observed in CRISPR-engineered flies and human epithelial cells whose Naked/NKD HisC has been deleted. Thus, HisC aggregation by Naked/NKD provides context-dependent feedback control of prolonged Wnt responses.